Plug for Tail Cone

Newest wrinkle in rocketry is the "plug nozzle" engine.

In the conventional rocket engine, fuel is burned in a roughly spherical combustion chamber, and turns into hot, high-pressure gas. To keep the gas from expanding wastefully in all directions as it leaves the nozzle, it is channeled into a tail cone where its pressure is efficiently converted into thrust as it expands (see diagram). The cone should be long enough to reduce the pressure of the gas to that of the surrounding atmosphere. Thus rockets intended to work at very high altitudes must have extra-long tail cones.

In the plug nozzle engine, the fuel is burned in a doughnut-shaped combustion chamber. The gas escapes from a ring-shaped nozzle surrounding the base of a conical plug. Instead of expanding sideways, the gas follows the plug toward its point. In effect, its expansion takes place in a flaring chamber bounded by the plug and the atmosphere.

Unlike the conventional engine, the plug nozzle works almost equally well at sea level and at high altitude. The ring-shaped combustion chamber can be divided into many small segments, each with its own fuel supply. By adding segments, the engine can be built in very large sizes without running into the difficulties that plague the designers of single combustion chambers. Single-chamber rockets are steered by mounting their engines on gimbals so that their thrust can be switched from side to side. With plug nozzle engines, the same control can be achieved by varying the fuel supply of one or more of the combustion chambers.

General Electric, Aerojet-General, Pratt & Whitney and Rocketdyne are all working on plug nozzles. Tests have been promising, and rocket men predict that for many applications the plugs will eventually supersede the graceful tail-cone engine.

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